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Saturday, November 1, 2008

Beached whale

A beached whale is a whale which has become stranded on land, usually on a beach. Beaching is often fatal for whales, as they become dehydrated and die. Some die when their lungs are suffocated under their own weight or drown when high tides cover their blowholes.

Species

Every year there may be beachings adding up to 2000 animals. Although the majority of strandings will result in death, they pose no threat to the species as a whole. Of all of the species of cetaceans, only about 10 species are frequently involved in mass beachings, with a further 10 species rarely being involved. All of the frequently involved species are toothed whales, meaning that none of the baleen whales are regularly involved in beachings. These species share some characteristics which may explain why they beach. Body size does not normally affect the frequency, but both the animals' normal habitat and social organization do appear to influence their chances of coming ashore in large numbers. Odontocetes that normally inhabit deep waters and live in large, tightly knit groups are the most susceptible. They include the Sperm whale, a few species of Pilot and Orca whales, a few beaked whales and some oceanic dolphins. Solitary species are naturally excluded from mass strandings. Cetaceans that spend most of their time in shallow, coastal waters are almost never mass stranded, with porpoises being essentially immune.

Causes

Overview

Strandings can be grouped into several different types. The most obvious distinctions are between single and multiple strandings. The carcasses of deceased cetaceans are likely to float to the surface at some point; during this time, currents or winds may carry them towards a coastline. Since thousands of cetaceans must die every year it is natural that many will become stranded. Most cetacean deaths which occur naturally will only involve a single individual, causing these strandings to consist of just one animal, if a stranding occurs at all; however, most carcasses never reach the coast and are scavenged or decomposed enough to sink to the bottom of the oceans. Single live strandings are often the result of an illness or injury, which would almost inevitably end in death unless a passer-by knows how to react.

Multiple strandings of dead animals in one locality are rare and often cause a great deal of media coverage as well as rescue efforts by sympathetic humans. Even multiple offshore deaths are unlikely to lead to multiple strandings due to the variable winds and currents scattering the animals across the sea. A key factor in many of these cases appears to be the strong social cohesion amongst toothed whales. If one whale gets into trouble, its distress calls may prompt the rest of the pod to follow and become beached themselves. There is no definitive specific cause for mass strandings. Instead they may happen due to numerous factors that can act in combination. Many have been proposed, with some seeking to explain only a subset of cases; however, they are difficult to prove conclusively and are sometimes controversial.

Natural

Whales have been found beached throughout human history so many strandings can be attributed to natural and environmental factors. There could be many natural reasons like rough weather, weakness due to old age or infection, difficulty giving birth, hunting too close to shore and navigational mistakes. A single stranded animal can prompt the entire pod to respond to its distress signals and become stranded. In 2004, scientists at the University of Tasmania found a link between whale strandings and the weather. It is hypothesised that when cool Antarctic waters rich in squid and fish flow north, whales follow their prey closer towards land making them more prone to stranding. In some cases predators (such as killer whales) have been known to panic whales, herding them towards the shoreline.

Another proposed cause is that the echolocation system used by many whales can have difficulty picking up very gently-sloping coastlines.[ This theory accounts for mass beaching hot spots such as Ocean Beach, Tasmania and Geographe Bay, Western Australia where the slope is about half a degree (approximately 8m deep one kilometer out to sea). The Bioacoustics group at the University of Western Australia has done research[ indicating that repeated reflections between the surface and ocean bottom in gently-sloping shallow water may attenuate sound so much that the echo is inaudible to the whales. Stirred up sand as well as long-lived microbubbles formed by rain may further exacerbate the effect.

A controversial theory, researched by Jim Berkland, a former geologist with the U.S. Geological Survey, attributes the strange behaviour to radical changes in the Earth's magnetic field just prior to earthquakes and in the general area of earthquakes. Berkland says when this occurs, it interferes with sea mammals' and even migratory birds' ability to navigate, which explains the mass beachings. He says even dogs and cats can sense the disruptions, which explains elevated rates of runaway pets in local newspapers a day or two before earthquakes occur. Research on Earth's magnetic field and how it is affected by moving tectonic plates and earthquakes is ongoing.

"Follow-me" strandings

Another possibility is that a proportion of strandings may be caused by larger cetaceans following dolphins and porpoises into shallow coastal waters[citation needed]. The larger animals may be familiar with faster moving dolphins in their area and become habituated to following them. If an adverse combination of tidal flow and seabed topography is encountered, the larger species are at much higher risk of being trapped.

Sometimes it may work the other way. A recent example of this was in where a local dolphin was followed out to open water by 2 Pygmy sperm whales that had become lost behind a sandbar at Mahia Beach, New Zealand. The possibility of training dolphins to lead trapped whales out to sea is an intriguing one in areas where frequent mass strandings occur, such as New England or Florida.

An interesting observation is that pods of killer whales, predators of dolphins and porpoises, are very rarely stranded. There may be two explanations for this - firstly that heading for shallow waters may be a very effective anti-predator mechanism for dolphins and that the killer whales have learned not to take the risk, or else that the killer whales have learned how to operate in shallow waters, particularly in their pursuit of seals. The latter is certainly the case in Península Valdés, Argentina, where a particular pod of killer whales pursue seals up shelving gravel beaches to the edge of the littoral zone[citation needed]. The pursuing whales are occasionally partially thrust out of the sea by a combination of their own impetus and retreating water and have to wait for the next wave to take them back out. Man-made

There is evidence that very loud noise from anti-submarine warfare sonar may hurt whales and lead to their beaching. On numerous occasions whales have been stranded shortly after military sonar was active in the area, suggesting a link. Reasons as to how sonar may cause whale deaths have also been put forward by scientists after necropsies found internal injuries in stranded whales. In contrast, whales stranded due to seemingly natural causes are usually healthy prior to beaching.

Firstly, it has been argued that the very loud rapid pressure changes made by sonar can cause haemorrhaging. Evidence emerged after the beachings of seventeen whales and a dolphin in the Bahamas in March 2000 following a United States Navy sonar exercise. The Navy accepted blame in this Joint Interim Report which found the dead whales to have experienced acoustically-induced haemorrhages around the ears. The resulting disorientation probably led to the stranding. Ken Balcomb, a zoologist, specializing in the study of whales, particularly the Orca populations that inhabit the Strait of Juan de Fuca between Washington and Vancouver Island, has studied mass beachings of whales. He investigated the March 2000 beachings and argues that the powerful sonar pulses had used vibrations that resonated with airspaces in the whales, causing tearing of tissue around the ears and brain.

Another means by which sonar could be hurting whales is a form of decompression sickness. This was first raised by pathological examinations after 14 beaked whales stranded in the Canary Islands. The stranding happened on 24 September 2002, close to the operating area of Neo Tapon (an international naval exercise) about 4 hours after the activation of mid-frequency sonar. team of scientists found acute tissue damage from gas-bubble lesions, which are indicative of decompressions sickness. precise mechanism of how sonar causes bubble formation is not fully known. It could be due to whales panicking and surfacing too rapidly in an attempt to escape the sonar pulses. There is also a theoretical basis by which sonar vibrations can cause supersaturated gas to nucleate to form bubbles.

Strandings may be caused by poisoning, as happened in 1987-88 to Bottlenose Dolphins off the north-eastern coast of the United States.

Carcass

After a beached whale dies, it can become a source of disease and pollution. Due to the very high efficiency as thermal insulator of the blubber, a whale carcass can keep its internal temperature over 30 celsius for up to 3 days, making it the ideal environment for anaerobic microorganisms.

As they are very large, such corpses are difficult to move. There are reports of some cases where humans tried to blow up the carcass with explosives, with unexpected side effects to spectators.

A whale carcass should not be consumed. In 2002, fourteen Alaskans ate muktuk (whale blubber) from a beached whale, and eight of them developed symptoms of botulism, two of them requiring mechanical ventilation. This is a possibility for any spoiled meat, or meat taken from an animal which has been dead for some time without proper preservation of the carcass.